There is a recently developed white light-emitting LED device that comprises a blue LED and a yellow phosphor Y3Al5O12:Ce3+ (YAG) in combination. For the purpose of expanding new markets, this device is studied for use as an illumination light or a backlight source of liquid crystal display. However, since the light emitted by the device is a mixture of blue emission from the blue LED and luminescence from the yellow phosphor, the device gives “pseudo-white” color light, which is lacking in red color. Accordingly, from the viewpoint of color rendition, there is room for improvement. In view of this, a new white light-emitting device is proposed that comprises a red light-emitting fluorescent substance in addition to the blue LED and the YAG phosphor, and the red light-emitting fluorescent substance used therein is being vigorously researched in these days.
Meanwhile, it has been more and more required for the white light-emitting device to be improved in luminance. To meet this requirement, increased electric power is often applied to the device and accordingly the device operating temperature is apt to rise considerably. When the device works at a high temperature, the emission efficiency generally drops that the expected luminance often cannot be obtained and/or that the luminance balance among the fluorescent substances may be lost to cause color discrepancies. Also from this point of view, it is desired to provide a fluorescent substance excellent both in luminance and in temperature characteristics.
Further, according to increased demand for high color rendition, various types of the white light-emitting device have been developed and commercially sold for lighting purposes or the like. The color rendition is often evaluated in terms of the general color rendering index (Ra), and hence it is desired to provide a high color rendering light-emitting device giving a high Ra value. On the other hand, for application to displays, it is desired to provide a white light-emitting LED device having both a wide gamut of reproducible colors (NTSC ratio) and a high efficiency.